The present invention relates to a method and apparatus for counting discrete small objects suspended in a flowing liquid.
There are well-developed systems for counting small objects of regular shape in dry systems. An example might be in packaging such items as screws, ball bearings, etc., which are more conveniently sold by number than by weight. This is usually done by singulating the objects on a conveyor and sensing them by means such as a photocell, magnetic transducer or the like. Objects that must be maintained wet or in a liquid present different and largely unaddressed problems of counting. Equipment is available for determining concentrations of relatively small percentages of extremely fine suspended particles. However, there has not been any wet system which gives a number count of anything that might be described as being of discrete object size.
While the counting problem described above occurs in many situations, one particular industry where it is of importance is aquaculture. Aquaculture may be defined as the cultivation of any animal living in an aquatic environment.
Fish eggs and fish fry are examples of products that are usually marketed by numbers. Most often the number count is merely an approximation based on volumetric or weight estimates.
Another product also sold commercially by number count is immature freshwater shrimp, often of the species Macrobrachium rosenbergii. These are shipped from hatcheries in a post-larval state when they are about 1 centimeter in length and weigh between 0.01 and 0.03 gram. Before the present invention, it was standard practice to count these delicate, squirming little creatures by hand. This is an expensive, tedious and time-consuming operation which is traumatic to the delicate crustaceans, often inducing considerable mortality. In addition, it does not give the accuracy that might be expected. At best, hand counting gives values which are probably in a range of .+-.5 percent of true counts.
As an example of existing technology, U.S. Pat. No. 3,879,129 to Inoue discloses a light scattering means for measuring the size and concentration of small particles up to diameters of several hundred microns. This method is apparently not effective with particles in the millimeter and larger size range; however, nor does it give a number count.